High-power radiotelephony



Patented Apr. 28, 1925.

NITED STATES PATENT OFFICE.

ARTHUR A. OSWALD, 0F EAST ORANGE, NEW JERSEY, lASSIGNORITO WESTERN ELEC- TRIC COMPANY, INCORPORATED, 0F NEW YORK, N.` Y., A CORPORATION 0F NEW YORK.

HIGH-POWER RADIO'TELEBHON Y.

Application filed March 20, 1924. Serial No. 700,557.

To all a0/10m t may concern.'

llc it known that I, ARTHUR A. OswALD, a citizen of the United States of America, residing at East Orange, in the county of llssex and State of New Jersey,-have invented certain new and useful Improvements in High-Power Radiotelephony, of which the following is a full, clear, concise, and exact description.

This invention relates to radio telephone transmission at frequencies adapted for long dista-nce communication.

Hithcrto, radio telephone communication has been largely accomplished by transmission at wave lengths of 100,000 cycles and upward and at relatively low powers. For long distance communication, it is deemed advisable for practical reasons to operate at frequencies below 100,000 cycles. In particular, in a series of experimental tests, in which the methods and. apparatus of the present invention were used, a frequency band centering around 57,000 cycles has been used.

Ordinary autennzc usually have a transmission band of insufficient width to radiate a band of frequencies of the width of the speech band and extending, for example, from 55,750 to 58,250 cycles. A certain antenna which was used had a transmission band of only 1100 cycles. It has been found that such an antenna can be associated with vacuum tube apparatus in such a manner as to provide an effective input-response frequency characteristic substantially uniform over a band width of 2500 cycles without `greatly reducing the operating efficiency of the vacuum tubes. It has developed that the problems involved in radiotransmission of a band of frequencies of 2,000 cycles or more at high efficiency are considerably more difficult than the problems involved in radiating a similar amount of energy in a band of frequencies so narrow as to constitute practically a singlefrequency, as in radio telegraph transmission at ordinary speeds. The solution of certain of these problems is effected by the apparatus andmethods herein described, but the invention does not inhere in any particular theory or explanation of supposed reasons for the results obtained. Hence, any theory or explanation advanced herein is to be considered asan expanation of the invention and not as a limitation thereof.

`power and transmit a One object of the invention is to provide a method of widening the transmission band of an antenna circuit by employing vacuum tubes in combination therewith and in conneet-ion with a coupling circuit so designed that the tubes assist in widening'the band.

A feature of the invention consists in a circuit so designed that the tubes work into an impedance which increases in value as the edges of the band are approached.

Further objects are to amplify to high radio Wave of speech band width. In general, the widening of the transmitted band of frequencies in this manner results in some decrease in efficiency as compared with the efficiency which might be secured with the same tubes in transmitting a current of a single frequency. The cause of this is that the application of the method herein described results in there being impressed on the anode circuit of the tubes increased electromotive forces. In the usual practical case, the application of thel method is restricted by the maximum voltage which the tubes will stand. This requires that, if the tubes are operating at maxlmum voltage prior to the application of the method, then upon the application of the method, the voltage resulting from the transmission of waves in the middle of the transmitted frequency band must be reduced.

A further object is to improve circuits for coupling power tubes to an antenna circuit. In carrying out the invention in practice, use has been made of a. group of approximately twenty water-cooled anode power tubes rated at kilowatts each and supplied with space current at a tension of approximately 10,000 volts. These tubes constitute the final stage of a power' amplifier. A tuned circuit couples the tubes to the aerial circuit which consists of a multiple directive antenna of a well-known type. Such antennae vare described in articles by Alexanderson and Bucher in the General Electric Review for October 1920 and an article by Alexanderson, Reoch and Taylor in the A. I. E. E. Proceedings for July 1923. Further details of the vacuum tube apparatus and circuits will be given hereinafter.

In operating, it has been found that not only can a fairly flat transmission characteristic be obtained but, if desired, portions of the transmitted wave band near the edges thereof can be so effectively converted into coA the electroma-gnetic radiation that unequal attenuation o speech input apparatus,-such as a long telephone line can be wholly or partly compensated for. h

"he invention will be described in greater detail in connection with the accompanying drawings.

Fic. 1 is a partly diagrammatic and partly sclhematic illustration ofthe circuit employed.V A

Fi 1a is a more detailed disclosure of a coup ing circuit.

Fig. 2 is a graph illustrating the widening of the transmission band.

Fig. 8 is a graph of the variation of the impedance of the power tube anode circuit with frequency. y

Referring to Fig. 1, the source of speech frequency waves 1 is connected to the input circuit of modulator 2 which is supplied with high frequency carrier waves from source 3. Filter 4 selects one side band fromvthe resultant modulated wave. The apparatus 1, 2, 3, 4 indicate any desired type of' combination of elements suitable for producing a side band of speech frequency width. The side band this produced is amplified in any desired number of stages by amplifiers 5, to the output circuit of which is connected a power tube amplifying system 6. The power tube amplifier 6 has its output circuit connected to 'a timed circuit 7 which is in turn coupled to a multiple tuned antenna 8. The system 6 issupplied with grid -voltage of a tension of 250 to 350 volts from a-source 9 through a suitable filter'lO. The cathodes'are heated from a 60 cycle or other commercial source of current 11. Anode current is supplied by a source 12 having a terminal voltage of 7,000 to 11,000 volts in accordance with the particular adjustment and power.

In order to couple the power amplifier system 6 with the aerial system 8, the coupling circuit 7 is provided. A capacity y13 functioning. as a stopping condenserl prevents short-circuiting of the source 12. The coupling circuit 7 comprises a variable inductance 15, a coupling inductance 16, and capacities 14 and 20. lThe coupling inductance 16 is coupled to the inductance coil 17 which is located in the down lead of the antenna circuit.

I The antenna circuit comprises a plurality of sections 18 connected to each other with inductance coils 19 extending from the points of `connection to ground.

Some of the requirements for the system are that, having given an amplifier capable of delivering a certain amount lof high frequency power and an antenna with certain characteristics, the intermediate circuitV must be desi ed to satisfy the impedance requirements oth of the power tubes and of the antenna. A second condition is that the power at all of the frequencies in the band must receive substantially the same amplification in assing through the system to the antenna. n order to revent interference, currents of harmonic rcquencies must not be allowed to flow in the antenna. frequency used the multiple resistance of the antenna is approximately .75 ohms. 'lhe radiation resistance, taking account of the directive effect, is little more than .3 ohms. The radiation efficiency is therefore more than 40%. Such a low antenna resistance is of itself desirable, but it results in a narrow resonance characteristic effect which for telephony, leads to certainy complications.

The band width is here arbitrarily taken as the difference between the two frequencies for which the impedance of the tuned antenna circuit is double the minimum value. It is particularly desirable to transmit a wider band than this where the system is -used with standard microphones, receivers ang relative long-line connections at each en The electrical design procedure consists of twol steps; first, a preliminary determination of certain constants on the basis of a single frequency, and second, an examination of the frequency-impedance characteristic throughout the Aband for the purpose of modifying the constants so as to obtain the most. practical inputresponse-frequency characteristic for the amplifier system.

Vhen properly adjusted it has been found that the output impedance may have two peaks with a relatively low point approximately half-way between, as shown in Fig. 3 which is not drawn to scale but is intended to show the general shape of the impedancefrequency characteristic. At 57 kilocycles the antenna introduces only resistance into the primary circuit. At lower frequencies positive reactance is introduced and at a somewhat lower point about 55.7 kilocycles the impedance passes through a maximum. For frequencies above 57 kilocycles capacity reactance is added to the inductive branch of the timed circuit7rnaking it anti-resonant at tlieffrequency around 58.3 kilocycles. Another important feature is that the power factor is approximately unity at three differ-` ent frequencies which occur at the middle and near the edges of the band. Furthermore, the impedances at the pointsA, B and C (Fig. 3) are not in any fixed relation but may be adjusted with respect r,to each other by adjustments ofthe circuit. In particular the impedances at the points B and C are capable of considerable 'regulation so that one peak may be considerably'higher than the other if desired. A characteristic feature of the invention, therefore, is that at frequencies near the edges ofthe transmitted band at which it is difficult to'transfcr energy into the antenna, the tubes are caused to work into an increased impedance. This lill) pedance of the tubes is zero,` we would be .applying to the output circuit a constant 'p voltage equal to p. timesthe alternating grid voltage, Where p. is the amplification constant of tube. The antenna current characteristic would not differ very much from4 the resonance. curve of the antenna. But when account is taken of the fact that the impedance of the tubes is finite, We find that the same potential is not applied to the output circuit for all frequencies but is -greatest for frequencies for which the output im pedance is greatest. Hence, there is a tend'- ency to depress the antenna current characteristic at points of low outputimpedance. Th effective band Width maythus be made several times as great as vthe resonance band of the antenna. The adjustmenty finally adopted should be one for which the efficiency of the amplifier is satisfactory.

In the application of this invention there must be taken into account the fact that vacuum tube constants, and, in particular, the plate circuit impedance of a vacuum tube are not the same when large voltages are impressed upon the grid circuit and the combination of high anode potential and high negative grid potential is employed as when small voltages are impressed upon the grid circuit and smaller anode potentials and grid polarizing potentials are employed. Vhen power tubes are employed under conditions selected with a lView to obtaining high efficiencies, the effective internal tube impedance normally of the order of 5,000

ohms per tube with a particular type of tube may be greatly increased.

The advantage of Widening the inputresponse frequency characteristic is accompanied by a certain necessary disadvantage vwhich is, namely, a diminishedfactor of safety. With increase of output impedance there is a corresponding, although not proportional, increase in the alternating component of the anode potential. Consequently, the anode may become negative by a value which may be as large as one-half the voltage of the direct current supply. T'here is no objection to this under ordinary circumstances, but in one-half of the cycle, since the Wave is symmetrical, the anode potential is of the order of two and one-half tiines the voltage supply. Hence, if the same factor of safety is to be maintained, a decreased rating for the powerv tubes must follow.

From the foregoing, it follows that radio telephone transmission may be accomplished upon an antenna with a narrow reasonance characteristic indicated by the curve l in Fig. 2 with a band width of 1,100 cycles or less with a resultant input voltage-response characteristic sufficiently uniform over a band of 2,500 cycles or more to permit of reasonably efficient transmission with a minimum -of distortion.4 It has been, in fact, found that an antenna With a band width of 600 cycles could be effectively used.- n Consideration must be given to the large increase of harmonics which results when the anode potentialassumes negative values during part of the cycle. These currents of hai-momo .frequencies must be kept out of the antenna..v `For high efficiencies it is consldered desirable that the first three or four multiple frequencycom onents pass through the condenserlff.v The higher harmonics and particularly those which occurwithin the roadcasting range at present employed, are dis osed of bya special shunt arrangement .to e described in connection with Fig. l,

In Fig. 1 a circuit 7 is illustrated in greater detail. The capacity 13 as employed 1n practice consists of a large bank of condensers arranged in a. multiple-series combination. In order to equalize the direct potential component on these condensers, each parallel gmup is shunted by high resistance, since otherwise the result of small current leaks would be to impress upon the highest1 resistance parallel group a disproportionately large part of the direct potential applied td the plate circuits.- In consequence of placing resistances 21 around the condensers 13 it becomes necessary to utilize some means for preventing the condensers 20 from receiving the entire 10,000 volts of the plate 'current supply generator 12. This may be done by connectingr similar high but p lower resistance 22 around-the condensers 20 or, if desired, by connecting a high impedance choke coil 23 from the right hand terminal of the condensers 13 to ground lby means of a switch 24. The coupling inductance 16 is sh'unted by a resistance 25 in series with acondenser 26. The impedance of the capacity 26 is such that it has little eiiect at the operating frequency or the lower harmonies, but at the higher harmonic frequencies Which fall within the broadcasting range Where little interference can be tolerated, currents are shuntcd through the resistance 25 and 26 and thus divcrtedvfrom the aerial circuit.

The novel features believed to be inherent in the invent-ion are set forth in the appended claims.

What is claimed is:

1. The method which comprises transmitting a band of frequencies of a total freuency range of the order of 2000 cycles t rough an antenna having a resonance characteristic materially narrower than 2000 Yti cyclels with the entire frequency range of tiveness over the band. l

2. A system comprising charge device having a cuit connected toa radiating antenna by means of a tuned couplingr circuit in combination with a negatively polarized control element in said space discharge tube, and an input means of sufficient power to cause said clement to become positive at times.

3. The method of widening the transmission band of an antenna having vacuum tube repeaters feeding into the antenna, which consists in operating the tubes in such a manner. that they assist in widening the band.

4. A radio transmission circuit forv transmitting by means of an antenna a band of frequencies relatively wide as compared with the Width of the resonance characteristic of the antenna which comprises a vacuum tube and a circuit coupled between the tube and the antenna, said circuit being so designed that the tube works into an impedance which increases in value as the edges of the band are approached.

5. A radio telephone transmission system comprising apparatus and circuit arrangements for producing a band of high frequency waves representing speech, al space a. thermionic disdischarge amplifier having a grid-cathode input circuit and an anode-cathode output circuit for amplifying the energy of the waves of said band, an adjustable tuned circuit connected to the anode-cathode circuit of said amplifier, and a multiple tuned antenna coupled to said adjustable circuit, the impedances of said adjustable circuit, of said antenna. and of said anode-cathode path being so proportioned as to transmit waves at the edge of the band of frequencies with` l an effectiveness equal to or greater than the transmission at the middle of said band.

6. In combination, a source of high voltage, a stopping condenser connected between said source and a coupling circuit, said stopping condenser comprisingcapacity elements shunted by resistance to prevent the break down of the capacity elements, a capacity in lsaid coupling circuit which is subjected to the voltage of said source as a result of leakage current through said resistance, said space discharge circapacity in said capacity elements in series relation s unted by resistance.

7. A circuit for preventing harmonics combination ,being high at the operating frequency and low at the frequencies of waves to be excluded from said antenna.

8. The method of widenin the transmission characteristic of an amp ifier delivering power to a tuned load circuit, which consists in coupling a primary circuit with said load circuit thereby introducing impedance into said primary circuit, said primary circuit being tuned to a frequency of the same order as .said load circuit, said primary circuit being coupled with the output circuit of said amplifier in such a way that a variation of frequency tending to producea decrease in the resistance of said primary circuit including that introduced due to said load circuit results in an increase in the impedance into which said amplier works.

- 9. A transmission system comprising a load circuit, a repeater constituting a source for supplying a relatively wide band of frequencies cuit comprising elements electrically selective of a portion of the frequencies of the band in the central zone thereof, in combination with a coupling circuit interposed between said source and saidload circuit, said coupling circuit consisting of impedance elements having their impedances so proportioned and so related to the impedance of said source and-of said load circuit that lthe impedance of said coupling circuit including the impedance introduced from said load circuit as viewed from said source is of conto said load circuit, said load cir.

coupling circuit com rising siderablv` greater numerical value at the edge frequencies of said band than in the central zone thereof, whereby greater uniformity of transmission of waves of frequencies within said band is attained.

In witness whereof, I hereuntosubscribe my naine this 7th day of March A. D., 1924.

ARTHUR A. OSWALD. 

